In a number of medical procedures, one part of a patient's body is anchored to a desired location on one of the patient's bones. The body part being anchored may be living tissue, such as a ligament or bone, or a prosthetic replacement of an original body part. In one such procedure, a dental prosthesis, such as a crown or bridge, is used to replace one or more of a patient's teeth. Such dental prostheses have been anchored to bone using an anchoring assembly which included a bone anchor, such as a female bone screw or other endosseous implant, for being embedded in the patient's jaw bone. One such bone anchor has an opening leading to a threaded bore and a coupling member adapted at one end for being threaded into the bone screw bore. In another anchoring assembly, the coupling member is cemented into the bore formed in the bone anchor. The other end of the coupling member is fixed to the dental prosthesis. Sometimes the crown fractures off of the tooth leaving its root intact and embedded in the patient's jaw bone. In such situations, the tooth root may be used as the bone anchor, with the coupling member cemented or otherwise fixed within the tooth's root canal. One problem with such anchoring assemblies is that if the coupling member has to be removed, such as to replace the dental prosthesis, a force must be applied to the coupling member which is transmitted to the bone anchor. For example, if the coupling member is threaded into the bone anchor, a sufficiently high torquing force must be applied in order to unscrew and remove the coupling member. If the coupling member is cemented to the bone anchor, a sufficiently high force must be applied to break the cement bond between the coupling member and bone anchor.
Such female bone screws, or other bone anchors used with a coupling member, are preferably first implanted and left in the patient's bone for a period of time before the coupling member is secured to the bone anchor and an external force is applied to the bone anchor. This is done to permit the bone time to grow and bond with the bone anchor, thereby increasing the strength of the anchorage to the bone. The bone is less likely to bond to the bone anchor if an external force is applied soon after the bone anchor is implanted. Even if this bond is allowed to strengthen, the force required to remove the coupling member may be so high as to cause any bond between the bone anchor and the bone to weaken or even fail. In addition, screwing the threaded coupling member into the bone anchor may also apply a force of sufficient magnitude to weaken or even break the bond between the bone anchor and the bone. If this bond weakens or fails, the coupling member will likely need to be removed from the bone anchor in order to limit any external force applied to the bone anchor. Limiting such external forces is necessary in order to increase the likelihood that the bone-to-anchor bond will increase in strength. This additional delay in anchoring the body part to bone may inconvenience and cause additional discomfort to the patient.
Other problems associated with such anchoring assemblies may include the risk of the bone fracturing during removal of the coupling member from the bone anchor. This may also occur during insertion, particularly when a threaded coupling member is used. When the bone anchor is implanted, a comparable volume of bone is removed essentially leaving a cavity which may act as a stress concentrator. For smaller bones, such as some jaw bones, the amount of bone removed may significantly weaken the remaining bone at the anchoring site, making the bone more susceptible to fracturing when insertion or extraction forces are applied to the coupling member. In addition, when threaded coupling members are used, there is the risk that the threads will bind up. If this occurs, greater forces may need to be applied in order to insert and extract the coupling member. These greater forces may in turn cause the coupling member to fracture, typically at the threads. In order to subject the patient to as little discomfort as possible, it is typically desirable to complete the replacement procedure as quickly as possible. However, if there are spatial constraints, such as those associated with the small confines of a patient's mouth, attaching and removing the coupling member to and from the bone anchor may take more time than is desirable.
Other bone anchoring systems have been used to attach soft body tissue to a patient's bone. For example, certain orthopaedic surgical procedures require that a ligament be reattached to bone. Such ligament reattachment may occur, for example, in the knee or elbow where spatial constraints are typical. Such soft tissue attachment has typically been accomplished in the past by directly securing the soft tissue to the bone with a suture. Various types of bone anchors, such as bone screws, have been used in the past to anchor the suture to bone. These anchors are generally made of bio-compatible stainless steel or similar non-corrosive metal such as titanium or chromium-cobalt alloys. Suture thread is attached to the anchor and the anchor is implanted directly into the bone.
When a patient's soft tissue, such as a torn ligament, must be reattached to bone, the torn end must be anchored to the bone and placed in close proximity to allow the ligament to grow into the bone mass and reattach itself. The closer the suture is anchored to the original point of attachment of the ligament, the better for body mechanics. Design limitations of prior art devices have been known to limit the optimal placement of the suture anchor and have lead to less efficient attachment of ligaments. Manipulation required to set the anchor into bone may impact upon the ability of the anchor to be used in locations having spatial constraints. The degree of manipulation required may prevent the use of less invasive surgical techniques, like arthroscopic surgery, and require the use of a more invasive technique to ensure that the anchor can be implanted at the optimal location. Some prior art anchors may just be unsuited for use in areas having spatial constraints. If a bone anchor is unsuitable for placement in the original location of ligament attachment, a nearby location having suitable space must be utilized. Another problem with such an anchoring device is that if the soft tissue ever detaches from the bone anchor or if the suture breaks, the screw may have to be removed and implanted again. Such reimplantation causes additional trauma to the bone which may ultimately result in the original bone anchoring site being unsuitable for subsequent anchoring.
Various anchoring systems have also been used to attach bone-to-bone, such as through the use of a bone plate, like that used in distraction osteogenesis devices or bone fixation plates. Typically, such devices are anchored to bone using screws or some other anchor implanted in the bone. These prior bone anchors typically must be removed from the bone if the device they are anchoring is to be removed or replaced. Again, the trauma to the bone caused by such reimplantation may result in the original bone anchoring sites becoming unsuitable for subsequent anchoring.
Therefore, there is a need for an anchoring system which enables a body part to be fixed to and detached from a patient's bone while eliminating, or at least substantially limiting, any additional trauma to the bone. More particularly, there is also a need for an anchoring system which if the body part is detached and reattached to the bone, the bond between the implanted anchor and the bone will not be substantially affected. There is also a need for such a bone anchoring system which may be anchored at locations on the bone that have spatial constraints using surgical procedures that are less intrusive and traumatic to the patient's body.